Method for dispensing reagent onto a substrate

ABSTRACT

A method and apparatus for dispensing precise quantities of reagents is disclosed including a positive displacement syringe pump in series with a dispenser, such as an aerosol dispenser or solenoid valve dispenser. The pump is controlled by a stepper motor or the like to provide an incremental quantity or continuous flow of reagent to the dispenser. The pump and dispenser are operated in cooperation with one another such that the quantity and/or flow rate of liquid dispensed by the dispenser can be precisely metered substantially independently of the particular operating parameters of said dispenser to attain a desired flow rate, droplet size or mist quality, droplet frequency and/or droplet velocity.

RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.10/459,381, filed Jun. 10, 2003, which is a continuation of U.S.application Ser. No. 09/286,154, filed Apr. 5, 1990, now U.S. Pat. No.6,576,295, which is a divisional of U.S. application Ser. No.08/899,325, filed Jul. 23, 1997, now U.S. Pat. No. 5,916,524, whichreissued as U.S. application Ser. No. 09/897,788, filed Jun. 29, 2001,now U.S. Pat. No. RE38,281 E, which is a continuation-in-part of U.S.application Ser. No. 08/687,711, filed Jul. 26, 1996, now U.S. Pat. No.5,738,728, U.S. application Ser. No. 08/686,957, filed Jul. 26, 1996,now U.S. Pat. No. 5,741,554 and U.S. application Ser. No. 08/687,712,filed Jul. 26, 1996, now U.S. Pat. No. 5,743,960. The entirety of eachone of these prior applications is hereby incorporated by referenceherein, and the priority of each of these prior applications is claimedin the present application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an improved method andapparatus for dispensing chemical reagents and other liquids onto asubstrate and, in particular, to various methods and apparatiparticularly adapted for dispensing precise quantities of chemicalreagents onto a receptive membrane, such as to form a diagnostic teststrip, having proved dynamic range of operation.

2. Description of the Related Art

Clinical testing of various bodily fluids conducted by medical personnelare well-established tools for medical diagnosis and treatment ofvarious diseases and medical conditions. Such tests have becomeincreasingly sophisticated, as medical advancements have led to many newways of diagnosing and treating diseases.

The routine use of clinical testing for early screening and diagnosis ofdisease or medical conditions has given rise to a heightened interest insimplified procedures for such clinical testing that do not require ahigh degree of skill or which persons may conduct on themselves for thepurpose of acquiring information on a physiological relevant condition.Such tests may be carried out with or without consultation with a healthcare professional. Contemporary procedures of this type include bloodglucose tests, ovulation tests, blood cholesterol tests and tests forthe presence of human chorionic gonadotropin in urine, the basis ofmodern home pregnancy tests.

One of the most frequently used devices in clinical chemistry is thetest strip or dip stick. These devices are characterized by their lowcost and simplicity of use. Essentially, the test strip is placed incontact with a sample of the body fluid to be tested. Various reagentsincorporated on the test strip react with one or more analytes presentin the sample to provide a detectable signal.

Most test strips are chromogenic whereby a predetermined solubleconstituent of the sample interacts with a particular reagent either toform a uniquely colored compound, as a qualitative indication of thepresence or absence of the constituent, or to form a colored compound ofvariable color intensity, as a quantitative indication of the amount ofthe constituent present. These signals may be measured or detectedeither visually or via a specially calibrated machine.

For example, test strips for determining the presence or concentrationof leukocyte cells, esterase or protease in a urine sample utilizechromogenetic esters which produce an alcohol product as a result ofhydrolysis by esterase or protease. The intact chromogenetic ester has acolor different from the alcohol hydrolysis product. The color changegenerated by hydrolysis of the chromogenetic ester, therefore provides amethod of detecting the presence or concentration of esterase orprotease, which in turn, is correlated to the presence or concentrationof leukocyte cells. The degree and intensity of the color transition isproportional to the amount of leukocyte esterase or HLE detected in theurine. See U.S. Pat. No. 5,464,739.

The emergence and acceptance of such diagnostic test strips as acomponent of clinical testing and health care in general has led to thedevelopment of a number of quality diagnostic test strip products.Moreover, the range and availability of such products is likely toincrease substantially in the future.

Because test strips are used to provide both quantitative andqualitative measurements, it is extremely important to provideuniformity in distribution of the reagents on the test strip substrate.The chemistry is often quite sensitive and medical practice requiresthat the testing system be extremely accurate. When automated systemsare used, it is particularly important to ensure that the test stripsare reliable and that the measurements taken are quantitativelyaccurate.

Application of one or more reagents to a test strip substrate is ahighly difficult task. The viscosities and other flow properties of thereagents, their reactiveness with the substrate or other reagents varyfrom reagent to reagent, and even from lot to lot of the same reagent.It is also sometimes necessary or desirable to provide precise patternsof reagent on the test strip having predetermined reagentconcentrations. For example, some test strips provide multiple testareas that are serially arranged so that multiple tests may be performedusing a single test strip. U.S. Pat. No. 5,183,742, for instance,discloses a test strip having multiple side-by-side detection regions orzones for simultaneously performing various tests upon a sample of bodyfluid. Such test strip may be used to determine, for example, levels ofglucose, protein, and the pH of a single blood sample. It is oftendifficult, however, to form sharp lines or other geometric shapes havinguniform concentrations of reagent.

For several years the industry has been developing dispensing methodsbased on the use of either air brush dispensers or solenoid valvedispensers. Air brushes use pressurized air flowing across a needlevalve opening to atomize the reagent into a mist which is then depositedonto the test strip substrate. The quality of the mist, reagentdispersion pattern and the amount of reagent flow onto the substrate iscontrolled by adjusting the needle valve opening and/or the pressure ofthe atomizing air flow. Solenoid valve dispensers generally comprise asmall solenoid-activated valve which can be opened and closedelectronically at high speeds. The solenoid valve is connected to apressurized vessel or reservoir containing the fluid to be dispensed. Inoperation, the solenoid is energized by a pulse of electrical current,which opens the valve for a predetermined duty-cycle or open time. Thisallows a small volume of liquid to be forced through the nozzle forminga droplet which is then ejected from the valve onto the targetsubstrate. The size and frequency of the droplets and the amount ofreagent flow onto the substrate is typically controlled by adjusting thefrequency and pulse-width of energizing current provided to the solenoidvalve and/or by adjusting the pressure of the reservoir.

Currently available dispensing methods, however, are limited in theflexibility they have to independently adjust and regulate the output ofthe dispenser in terms of droplet size or mist quality, droplet velocityand flow rates of dispensed reagent. Flow rates can often drift due tochanges in temperature or the viscosity of the reagent. This can causeundesirable lot to lot variances of reagent coating concentrations orcoating patterns. Many reagents that are used for diagnostic testing areso reactive with the receptive membrane or substrate that large dropletscan form impressions on the membrane surface at the point of initialcontact before the droplets flow together to form the desired pattern.As a result, it is sometimes desirable to dispense a fine mist or verysmall droplets of reagent onto the substrate. Often, however, a desireddroplet size or mist quality is simply not attainable for a desiredproduction flow rate. It is sometimes necessary, therefore, to performproduction runs of test strips at slower than optimal speeds in order toensure adequate results. This can increases the cost of productionsignificantly. Certain dispensers, such as solenoid valves, are alsosusceptible to clogging by small air or gas bubbles forming in the valveitself or in the lines or conduits which supply reagent or other liquidsto the dispenser. This is a major reliability problem with manyconventional solenoid valve dispensers.

While some of these problems can be controlled or mitigated by addingsurfactants or various other chemical additives to modify the surfacetension or other flow characteristics of the droplets, compatiblechemistry is not available for all reagents. Also the use of surfactantsand other chemicals can often lead to other problems either in the teststrip itself or in the dispensing apparatus or production processes.

SUMMARY OF THE INVENTION

The reagent dispensing method and apparatus in accordance with thepresent invention can dispense desired quantities of chemical reagentsor other liquids onto a substrate, such as a receptive membrane, whileadvantageously providing the ability to independently and preciselyadjust droplet size or mist quality, droplet velocity and reagent flowrates, both in terms of per unit time or per unit distance. Thus, thepresent invention provides new devices and methods of dispensing precisequantities of liquids having improved performance and dynamic range ofoperation.

In accordance with one preferred embodiment the present inventioncomprises an improved apparatus for dispensing precise quantities ofliquid onto a substrate. The apparatus comprises a dispenser having aninlet and an outlet and being adapted to form droplets of liquid havinga predetermined size and/or quality. The droplets are emitted by thedispenser so as to be deposited onto a receptive substrate. A positivedisplacement pump is provided in series with the inlet of the dispenserfor metering predetermined quantities of liquid provided to thedispenser. In this manner, the quantity and/or flow rate of liquiddispensed by the dispenser can be precisely metered substantiallyindependently of the particular operating parameters of the dispenser.

In accordance with another preferred embodiment the present inventioncomprises a method or apparatus for dispensing a reagent onto asubstrate. A positive displacement syringe pump is provided in serieswith a reagent dispenser. The pump is controlled via a stepper motor orthe like to provide precision incremental or continuous flow of reagentto the dispenser. The dispenser is selectively operated to form dropletsor a mist of droplets of a predetermined droplet size and/or qualitywhich are then deposited onto the target substrate. Advantageously, thedroplet size, mist quality, droplet velocity and/or flow rate of thereagent can be precisely controlled independently of the particularsystem operating parameters of the dispenser.

In accordance with another preferred embodiment the present inventioncomprises an apparatus for dispensing a liquid onto a substrate,comprising a dispenser having an inlet and an outlet and a valve adaptedto be opened and closed at a predetermined frequency and duty cycle toform droplets which are deposited onto the substrate. A positivedisplacement pump, such as a stepper-motor-operated syringe pump, ishydraulically arranged in series with the inlet of the dispenser formetering predetermined quantities of liquid to the dispenser. The pumpand dispenser are operated in cooperation with one another such that thequantity and/or flow rate of liquid dispensed by the dispenser can beprecisely metered substantially independently of the particularoperating parameters of said dispenser. In this manner, the size,frequency, and velocity of droplets dispensed by said dispenser can eachbe adjusted substantially independently of the quantity and/or flow rateof liquid being dispensed.

In accordance with another preferred embodiment the present inventioncomprises an apparatus as described above in combination with a carriageadapted for X, X-Y or X-Y-Z motion relative to the dispenser. Thedispenser and carriage are arranged and controlled in a coordinatedmanner to form droplets of reagent, ink, liquid toner or other liquid inaccordance with a predetermined desired matrix or pattern. If desired,an array of dispensers and associated positive displacement pumps may beprovided and the outlets of the dispensers being arranged in a desiredpattern suitable for attaining a desired print matrix or dot pattern.

These and other embodiments and modes of carrying out the presentinvention will be readily ascertainable from the following detaileddescription of the preferred modes, having reference to the attacheddrawings, the invention not being limited to any particular referredembodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic drawing of a precision metered dispensingapparatus having features in accordance with the present invention;

FIG. 1B is a schematic drawing of an alternative embodiment of aprecision metered dispensing apparatus particularly adapted forcontinuous web production operation and having features in accordancewith the present invention;

FIGS. 2A and 2B are cross-sectional and detail views, respectfully, ofan air brush dispenser having features in accordance with the presentinvention;

FIG. 2C is a graphical representation of the test strip substrate ofFIG. 2B illustrating surface concentration of dispensed reagent and theresulting concentration absorbed reagent;

FIG. 3 is a cross-sectional view of a solenoid valve dispenser havingfeatures in accordance with the present invention;

FIG. 4 is a cross-sectional view of an optional piezoelectric dispenserhaving features in accordance with the present invention;

FIG. 5 is a cross-sectional detail view of the syringe pump of FIG. 1;

FIG. 6 is a graph comparatively illustrating the range of flow ratesattainable with a precision metered aerosol dispensing apparatusconstructed and operated in accordance with the present invention;

FIG. 7 is a schematic drawing illustrating two possible modes ofoperation of a solenoid valve dispenser constructed and operated inaccordance with the present invention;

FIG. 8 is a schematic view of an electrostatic printer for use inaccordance with one embodiment of the present invention; and

FIG. 9 is a front elevational view of an optional dispenser platform andmulti-head dispenser for use in accordance with one embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1A is a schematic drawing of a precision metered dispensingapparatus 10 having features in accordance with the present invention.The dispensing apparatus 10 generally comprises a dispenser 12 fordispensing reagent 14 from a reservoir 16 and a positive displacementsyringe pump 22 intermediate the reservoir 16 and the dispenser 12 forprecisely metering the volume and/or flow rate of reagent dispensed. Thedispenser 12 is operated to provide individual droplets or a spray ofreagent, as desired, at the predetermined incremental quantity or flowrate.

FIG. 1B is a schematic drawing of an alternative embodiment of aprecision metered dispensing apparatus 10′ particularly adapted forcontinuous web production operation and having features in accordancewith the present invention. For convenience of description and ease ofunderstanding like reference numerals are used to refer to likecomponents previously identified and described in FIG. 1A. Thedispensing apparatus 10′ generally comprises a dispenser 12′ fordispensing reagent 14′ from a reservoir 16′. As described above, thedispenser 12′ can be selectively operated to provide individual dropletsor a spray pattern of reagent, as desired, at the predeterminedincremental quantity or metered flow rate.

In this case, however, tandem positive displacement syringe pumps 22 a,22 b are disposed intermediate the reservoir 16′ and the dispenser 12′for precisely and continuously metering the volume and/or flow rate ofreagent dispensed. The pumps 22 a, 22 b are preferably connected inparallel, as shown, and are isolated from one another by appropriatecheck valves 24′ such that each syringe pump 22 a, 22 b is capable ofindependently metering a volume and/or flow rate of reagent to bedispensed. This particular dispensing apparatus configuration hassignificant advantages for continuous web production applications sincethe syringe pumps 22 a, 22 b can be operated in alternating successionwhile allowing the non-dispensing syringe pump to draw additionalreagent 14′ from the reservoir 16′. In this manner, continuous webproduction is facilitated without interruption. Of course, one or moreadditional syringe pumps may also be used in a similar manner, ifdesired, such as for dispensing a wash fluid or other suitable reagentsor fluids. Alternatively, if desired, one or more continuous positivedisplacement pumps, such as a peristaltic pump, may be used forcontinuous web production.

The dispensers 12 and 12′, described above, may comprise any one of anumber of suitable dispensers well known in the art for dispensing aliquid, such as an air brush dispenser, a solenoid valve dispenser or apiezoelectric dispenser. Several particularly preferred examples aredescribed below for illustrative purposes. Those skilled in the art willreadily appreciate that a wide variety of other suitable dispensers mayalso be used to achieve the benefits and advantages taught herein.

Air Brush Dispenser

FIGS. 2A and 2B are cross-sectional and detail views, respectfully, ofan air brush dispenser 12 a for use in accordance with one embodiment ofthe present invention. The dispenser 12 a generally comprises a nozzleportion 32 a and a manifold portion 34 a. The manifold 34 a allowscompressed air to enter into a first annular chamber 36 a and allowsreagent to enter into a second annular chamber 38 a formed between aneedle valve 40 a and a corresponding orifice 42 a. The needle valve 40a is fitted within and extends through the orifice 42 a, as shown. It ispreferably axially adjustable in accordance with well-known needle valveadjustment techniques. The position of the needle valve 40 a relative tothe orifice 42 a determines the effective size of the resulting needlevalve opening 43, and thus the amount of reagent flow for a givenpressure differential.

Pressurized air flows over the needle valve opening 43 creating aventuri effect which draws reagent through the orifice 42 a onto the tipof the needle valve 40 a. The pressurized air accelerates past theorifice 42 a and the needle valve opening 43 over the tip of the needle40 a. The resulting high velocity air atomizes the reagent 14 flowingdown the needle 40 a. This creates an aerosol mist 45 which is ejectedfrom the nozzle 32 a along with the excess airflow. In a conventionalair brush dispenser, the volume of reagent dispensed by the nozzle 32 ais determined by the pressure differential of the compressed air sourcerelative to atmospheric pressure, the size of the needle valve opening43, and the viscosity and other flow characteristics of the reagent 14.

In accordance with one embodiment of the present invention, however, apositive displacement pump 22 is provided in series between thereservoir 16 and the air brush 12 a as shown in FIG. 1. The orifice 42 anow admits a flow of reagent as determined solely by the positivedisplacement pump 22. The reagent is ejected out of the orifice opening42 a and mixes with the pressurized air flowing out of the nozzle 32 a.Advantageously, in accordance with the present invention absolute volumeor flow rate is an input parameter controlled by the metering pump,rather than an output parameter which must be calibrated by trial anderror adjustment. Thus, the air brush can be used to deliver precisequantities and flow rates of reagent onto a test strip substrate. Thissubstrate is preferably a receptive membrane adapted to bond with thereagent so as to form a diagnostic test strip. However, the substrate 30may also be paper, celluous, plastic or any wet or dry surface capableof receiving a dispensed reagent or other liquid.

As discussed in more detail below, a reagent dispensing apparatus andmethod using the combination of an air brush dispenser and a meteringpump provides a new dimension of control which provides additionalproduction capabilities not achievable with conventional air brushdispensers. Unlike conventional methods of operating an air brushdispenser, which typically provide only a single stable operating pointfor a given input air pressure and needle valve opening, the method andapparatus of the present invention provides a wide range of metered flowrates for achieving a stable dispersion pattern. The limits of thisrange can be determined experimentally. An even wider range ofproduction flow rates can be achieved using a single pressure settingand a series of adjustable orifice openings as illustrated in FIG. 6,discussed later.

FIG. 2C is a graphical representation of the test strip membrane 30 ofFIG. 2B, illustrating surface concentration 46 of dispensed and theresulting concentration gradients 48 of the absorbed reagent in themembrane 30. For stable dispersion patterns, the surface reagentconcentration 46 assumes a standard Gausian distribution, as shown. Thewidth or standard deviation of the distribution pattern will depend uponthe shape of the dispersion pattern created by the nozzle 32 a (FIG.2B). This is dependent primarily on the shape of the exit nozzle 32 a,the needle valve 40 a and the input air pressure. Higher input pressureswill generally result in wider dispersion patterns.

Solenoid Valve Dispenser

FIG. 3 is a cross-sectional view of a solenoid valve dispenser 12 b foruse in accordance with another embodiment of the present invention.Solenoid valve dispensers of this type are commonly used for ink-jetprinting and are commercially available from sources such as The LeeCompany of Westbrook, Conn. The dispenser 12 b generally comprises asolenoid portion 32 b and a valve portion 34 b. The solenoid portion 32b comprises an electromagnetic coil or winding 36 b, a static core 38 band a movable plunger 40 b. The static core 38 b and movable plunger 40b are disposed within a hollow cylindrical sleeve 41 and are preferablyspaced at least slightly away from the inner walls of the sleeve 41 soas to form an annular passage 42 b through which the reagent or otherliquid to be dispensed may flow. The static core 38 b and movableplunger 40 b are preferably formed of a ferrous or magnetic material,such as iron, and are separated by a small gap 44. Those skilled in theart will appreciate that when the solenoid coil 36 b is energized amagnetic field is created which draws the plunger 40 b upward toward thestatic core 38 b, closing the gap 44 and opening the valve 34 b.

The valve portion 34 b comprises a valve seat 52, having an orificeopening 54, and a stopper 56 having a valve face 58 adapted to sealagainst the valve seat 52. The stopper 56 is in mechanical communicationwith the plunger 40 b and is spring biased toward the valve seat 52 viacoil spring 60. Again, those skilled in the art will readily appreciatethat as the plunger 40 b moves up and down, the valve 34 b will open andclose, accordingly. Moreover, each time the valve 34 b opens and closes,a volume of liquid is forced through the valve orifice 54 to form apulse or pressure wave which ejects a droplet of liquid from the exitorifice 61 of the nozzle tip 59.

Conventionally, a pressurized reservoir (not shown) having apredetermined constant pressure is used to force liquid reagent or otherliquid through the valve orifice 54 during the time interval in whichthe valve 34 b is open. Under controlled conditions, such dispensers mayhave a repeatability of ±2% with a minimum drop size of about 30-35nanoliters. The size of the droplet will be determined by systemoperating parameters such as the reservoir pressure, valve open time orduty-cycle, and the viscosity and other flow characteristics of theparticular reagent or liquid being dispensed. Of course, certain fixedparameters, such as the size and shape of the nozzle 59, will also playan important role in the operational characteristics of the valve interms of droplet size and repeatability. In general, however, dropletsize will increase with increasing reservoir pressure and valve opentime.

In accordance with the present invention, however, a positivedisplacement pump 22 is provided in series between the supply reservoir16 and the solenoid valve dispenser 12 b, as shown in FIG. 1. For agiven range of flow rates the valve orifice 54 (FIG. 3) now admits aquantity and/or flow rate of reagent as determined solely by thepositive displacement pump 22. For example, the flow rate could be setto deliver 1 microliter per second of reagent. The pump 22 will thendeliver a steady flow of reagent to the solenoid valve dispenser 12 b atthe programmed rate. As the solenoid valve is opened and closed, aseries of droplets will be formed at the desired volume flow rate andejected onto the target substrate 30. This substrate is preferably areceptive membrane adapted to bond with the reagent so as to form adiagnostic test strip. Alternatively, the substrate 30 may be paper,celluous, plastic or any other wet or dry surface capable of receiving adispensed reagent or other liquid.

Advantageously, within a certain operating range the size of thedroplets can be adjusted without affecting the flow rate of reagentsimply by changing the frequency of the energizing pulses 13 provided tothe solenoid valve dispenser 12 b. Of course, there are physicallimitations of valve open time or duty-cycle necessary to achieve stabledroplet formation. If the open time is too short relative to the flowrate provided by the metering pump 22, the pressure will increase andpossibly prevent the valve from opening or functioning properly. If theopen time is too long relative to the flow rate, then drop formation maynot be uniform for each open/close cycle. Nevertheless, for a given flowrate of reagent provided by the pump 22 there will be a range ofcompatible frequencies and/or valve open times or duty-cycles in whichstable dispensing operations may be achieved at the desired flow rateand droplet size. This range may be determined experimentally for agiven production set up.

Another significant advantage of the present invention is that thevelocity of individual droplets can be independently adjusted withoutaffecting the flow rate of reagent or droplet size. This can beaccomplished, for example, by varying the duty cycle of the energizingpulses 13 provided to the solenoid valve dispenser 12 b.

For example, at a drop volume of 83.3 nL the drop can be formed by using20 syringe steps with 1 valve opening using a 100 μL syringe with a24,000 step resolution. Using an open time of 5% will result in a higherdrop velocity than using an open time of 7%. This is because with theshorter open time the pressure build up in the hydraulic line is greaterthan 7%.

Again, there are physical limitations posed by the length of duty-cyclenecessary to achieve stable droplet formation, as noted above.Nevertheless, for a given flow rate and droplet size there will be arange of compatible duty-cycles in which stable dispensing operationsmay be achieved at the desired flow rate, droplet size and velocity.Again, this range may be determined experimentally for a givenproduction set up.

As discussed in more detail below, dispensing a reagent by using acombination of a solenoid valve dispenser and a metering pump provides anew dimension of control which provides additional productioncapabilities not achievable with conventional solenoid valve dispensers.Unlike conventional solenoid valve dispensers, which typically have onlya single flow rate or operating point for a given set of systemoperating parameters (e.g. reservoir pressure, valve frequency and dutycycle), the present invention provides a wide dynamic range of meteredflow rates, droplet size, droplet frequency and droplet velocity forachieving stable dispensing operation. Moreover, because the solenoidvalve dispenser 12 b is forced to deliver precise quantities and/or flowrates of reagent, the solenoid valve dispenser is not as susceptible toclogging due to air or gas bubbles. Rather, any air or gas bubbles tendto be recondensed or ejected out of the solenoid valve dispenser 12 b byoperation of the positive displacement pump 22.

Piezoelectric Dispenser

FIG. 4 shows a cross-sectional view of an optional piezoelectricdispenser 12 c which may also have advantageous use in accordance withthe present invention. The piezoelectric dispenser generally comprises acapillary tube 84 made of glass or other suitable material and apiezoelectric constrictor 86 disposed around the capillary tube 84, asshown. The capillary tube 84 has a nozzle portion 88 of a reduceddiameter. When the capillary tube 84 is constricted by the piezoelectricconstrictor 86, droplets 90 are formed at the exit orifice 89 of thenozzle portion 88. Advantageously, the dynamics of the piezoelectricdispenser 12 c are such that it is able to operate at higher frequenciesand shorter duty cycles than typical solenoid valve dispensers,resulting in even smaller droplets 90. Operation of the piezoelectricdispenser in terms of adjusting droplet size, frequency, velocity andflow rates is substantially the same as that described above inconnection with the solenoid valve dispenser 12 b of FIG. 3 and,therefore, will not be repeated here.

Syringe Pump

A positive displacement pump for use in accordance with one particularembodiment of the present invention may be any one of several varietiesof commercially available pumping devices for metering precisequantities of liquid. A syringe-type pump 22, as shown in FIGS. 1A and1B, is preferred because of its convenience and commercial availability.A wide variety of other pumps may used, however, to achieve the benefitsand advantages as disclosed herein. These may include, withoutlimitation, rotary pumps, peristaltic pumps, squash-plate pumps, and thelike. As illustrated in more detail in FIG. 5, the syringe pump 22generally comprises a syringe housing 62 of a predetermined volume and aplunger 64 which is sealed against the syringe housing by O-rings or thelike. The plunger 64 mechanically engages a plunger shaft 66 having alead screw portion 68 adapted to thread in and out of a base support(not shown). Those skilled in the art will readily appreciate that asthe lead screw portion 68 of the plunger shaft 66 is rotated the plunger64 will be displaced axially, forcing reagent from the syringe housing62 into the exit tube 70. Any number of suitable motors or mechanicalactuators may be used to drive the lead screw 68. Preferably, a steppermotor 26 (FIG. 1) or other incremental or continuous actuator device isused so that the amount and/or flow rate of reagent can be preciselyregulated.

Suitable syringe pumps are commercially available, such as the Bio-DotCV1000 Syringe Pump Dispenser, available from Bio-Dot, Inc. of Irvine,Calif. This particular syringe pump incorporates an electronicallycontrolled stepper motor for providing precision liquid handling using avariety of syringe sizes. The CV1000 is powered by a single 24 DC voltpower supply and is controlled via an industry-standard RS232 or RS485bus interface. The syringe pump may have anywhere from 3,000-24,000steps, although higher resolution pumps having 48,000 steps or more mayalso be used to enjoy the benefits of the invention herein disclosed.Higher resolution pumps, such as piezoelectric pumps, may also be usedto provide even finer resolutions as desired. The lead screw 68 mayoptionally be fitted with an optical encoder or similar device to detectany lost steps. Alternatively, the lead screw of the metering pump canbe replaced with a piezoelectric slide to provide both smaller volumeincrements and also faster acceleration/deceleration characteristics.Multiple syringe pumps may also be used in parallel, for example, fordelivering varying concentrations of reagent and/or other liquids to thedispenser or for alternating dispensing operations between two or morereagents. This could have application, for instance, to ink jet printingusing one or more colored inks or liquid toners.

The travel of the plunger 64 is preferably about 60 mm. Plunger speedsmay range from 0.8 seconds per stroke with a 10-step minimum forlow-resolution pumping or 1.5 seconds per stroke with a 20-step minimumfor high-speed resolution pumping. The stroke speed may vary dependingupon the syringe size and the tubing used. Syringes may vary from lessthan 50 microliters to 25 milliliters, or more as needed. For mostreagent dispensing applications it should be adequate to provide asyringe having a volume from about 500 microliters to about 25milliliters. The minimum incremental displacement volume of the pumpwill depend on the pump resolution and syringe volume. For example, fora syringe housing volume of 500 microliters and 12,000 step resolutionpump the minimum incremental displacement volume will be about 42nanoliters. Minimum incremental displacement volumes from about 2.1nanoliters to 2.1 milliliters are preferred, although higher or lowerincremental displacement volumes may also be used while still enjoyingthe benefits of the present invention.

The syringe housing 62 may be made from any one of a number of suitablebio compatible materials such as glass, Teflon 198 or Kel-F. The plunger64 is preferably formed of virgin Teflon™. Referring to FIG. 1, thesyringe is connected to the reservoir 16 and the dispenser 12 using aTeflon tubing 23, such as ¼-inch O.D. tubing provided with luer-typefittings for connection to the syringe and dispenser. Various checkvalves 24 or shut-off valves 25 may also be used, as desired or needed,to direct the flow of reagent to and from the reservoir 16, syringe pump22 and dispenser 12 c.

Reagent Reservoir

The reagent reservoir 16 may be any one of a number of suitablereceptacles capable of allowing a liquid reagent 14 to be siphoned intopump 22. The reservoir may be pressurized, as desired, but is preferablevented to the atmosphere, as shown, via a vent opening 15. Theparticular size and shape of the reservoir 16 is relatively unimportant.

A siphon tube 17 extends downward into the reservoir 16 to a desireddepth sufficient to allow siphoning of reagent 14. Preferably the siphontube 17 extends as deep as possible into the reservoir 16 withoutcausing blockage of the lower inlet portion of the tube 17. Optionally,the lower inlet portion of the tube 17 may be cut at an angle or haveother features as necessary to desirable to provide consistent andreliable siphoning of reagent 14.

Operation

As indicated above, a key operational advantage achieved by the presentinvention is that over a certain dynamic range the flow of reagent,droplet size or mist quality, droplet frequency, and/or droplet velocitymay be controlled substantially independently of one another and of theparticular flow characteristics of the reagent and operating parametersof the dispenser 12. For example, the size of droplets formed by thedispenser can be adjusted without affecting the flow rate of reagentmetered by the pump by changing the operating frequency (for solenoidvalve or piezoelectric dispenser) or by adjusting the exit orifice size(for an air brush dispenser). The quantity or flow rate of reagentdispensed is substantially unaffected because it is precisely controlledby the positive displacement pump 22. This has particular advantage, forexample, in applications requiring the dispensing of very small dropletsor for dispensing higher viscosity reagents, since the reagent flow canbe precisely controlled without substantial regard to the systemoperating parameters otherwise required to achieve stable dispensingoperations. FIG. 6 comparatively illustrates the range of flow rates andoperating conditions for given orifice openings attainable in accordancewith the present invention using an air brush dispenser, versusconventional dispensing methods using an air brush dispenser.

Similarly, with a conventional solenoid valve dispenser in order toobtain very small droplets, one must attempt to shorten the open time orduty cycle of the valve. However, as the valve open time is shortened,the flow rate of reagent decreases such that the cycle frequency of thevalve must be increased to compensate. At a certain point the flowcharacteristics of the reagent will limit the ability to achieve uniformformation of droplets when the valve open time is very small. Moreover,even if stable dispensing operation could be achieved by increasing thereservoir pressure, such increased pressure will tend to increase thedroplet size and flow rate of reagent, necessitating even furtheradjustments to achieve stable dispensing operation at the desired flowrate and droplet size.

The present invention, however, overcomes these and other problems ofthe prior art by precisely metering the quantity and/or flow rate of thereagent. Advantageously, the amount of reagent can be preciselyregulated over a wide dynamic range without being substantially affectedby the particular operating parameters of the dispenser. This featureenables droplet size, droplet frequency, droplet velocity and othersystem parameters to be varied dramatically from one range to another ata given flow rate. Thus, the present invention not only provides amethod for precise metering of reagent, but also adds a new dimension ofoperation a dispenser not before possible.

Another important operational advantage is that the range of dropletsizes attainable with the present invention is much wider than achievedwith conventional solenoid valve dispensers. The method and apparatus ofthe present invention using the solenoid valve dispenser, for example,is capable of attaining minimum stable droplet sizes in the range of 1-4nanoliters, compared with 30-35 nanoliters for most conventionalsolenoid valve dispensers. In principle, even smaller droplet sizes (onthe order of 0.54 nanoliters or smaller) should be attainable inaccordance with the present invention using syringe pumps having aresolution of 48,000 steps and a syringe volume of 25 microliters. Dropformation experiments have demonstrated the ability to dispense4.16-nanoliter drops with very good repeatability using a nozzle 59(FIG. 3) having an exit orifice 61 of about 175 microns in diameter. Asmaller exit orifice 61 having a diameter in the range of 75-125 micronsshould provide stable formation and dispensing of even smaller dropletsin accordance with the present invention.

On the other hand, with the same setup one can program drop sizes orvolume delivered up into the range of 1 μL by pulsing the syringe manytimes per valve opening and by increasing the valve open time to allowthe larger volume to flow through the open valve. For example, for adrop size of 4.16 nL, the preferred setting would be 1 syringe step, 1valve opening and the open time would be 2% or about 0.2 millisecond.For a drop size of 1,000 nL or 1.0 μL, the preferred setting would be240 syringe steps, 1 valve opening and the open time would be in therange of 25%-30% or 2.5 to 3.5 milliseconds. One can also deliver thelarger volumes in a high frequency burst of smaller drops. For example,one can deliver 4.16 μL as 100 drops of 41.67 nL each using a frequencyof 100 Hz and an open time of 6% or 0.6 milliseconds.

Thus, the range of droplet sizes attainable for stable dispensingoperation may vary by a factor of about 250 or more. This feature of thepresent invention has particular advantage for high productionmanufacturing and processing of diagnostic test strips. In certainproduction applications, for example, it may be desirable to dispensevery small droplets or fine mists of reagent to provide optimal coatingcharacteristics. At the same time, it may be desirable to provide highreagent flow rates for increased production levels. With a conventionalsolenoid valve dispenser, for example, to increase the output flow ratethe valve frequency or the length of the valve open time must beincreased. But the longer the valve open time is, the larger thedroplets will be. There is also an operational limit for a given valveand exit orifice to how short the open time of the valve can be and howhigh the operating frequency can be while still attaining stableoperation.

The present invention, however, allows the use of much shorter valveopen times to attain stable operation at high flow rates by positivelydisplacing the reagent through the valve opening. In other words, theflow of reagent is not substantially affected by the particularoperating frequency of the valve or the length of the open time. It isdependent only on the displacement of the syringe pump, which acts asthe forcing function for the entire system.

Of course, as noted above, there will be a maximum range of operationfor a solenoid valve dispenser operating at given operating frequencyand valve open time. The higher limit will be the maximum amount ofreagent that can be forced through the valve at maximum design pressurefor the given operating frequency and valve open time. The lower limitwill be determined by the stability of droplet formation. If the valveopen time and/or operating frequency are too small for a given flowrate, the pressures in the dispenser will become too great, causingpossible rupture or malfunction of the system. If the valve open timeand/or operating frequency are too large for a given flow rate, the dropformation may not be uniform for each open/close cycle. Nevertheless,for a given flow rate of reagent provided by the pump 22 there will be arange of compatible frequencies and/or valve open times for which stableoperation may be achieved. This range may be determined experimentallyby adjusting the operating frequency and open time of the valve toachieve stable droplet formation. Similar advantages can be achievedwith air brush dispensers or other types of dispensers.

X-Y-Z Dispensing Platform

In a particularly preferred mode of operation, a dispenser may beintegrated to an X, X-Y, or X-Y-Z platform wherein the programmed motioncontrol can be coordinated with the metering pump to deliver a desiredvolume per unit length, with the ability to also independently controlthe frequency and droplet size of the reagent being dispensed. Forexample, it is possible to deliver reagent at a rate of 1 microliter percentimeter at a constant table speed with a droplet size ranging between4 and 100 nanoliters. The droplet size for a given dispenser flow ratecan be controlled by adjusting the operation frequency of the solenoidvalve. In this context, there are several particularly desirable modesof operation: (1) line or continuous dispensing; (2) spot or “dot”dispensing; (3) aspirating; and (4) dot matrix printing. Each of thesepreferred modes of operation is addressed below:

Continuous Dispensing

In the continuous dispensing mode, the metering pump is set to aprescribed flow rate to deliver a metered volume of reagent involume-per-unit time. For example, the flow rate could be programmed todeliver 1 microliter per second. The syringe will then pump reagent tothe solenoid valve 12 at the predetermined rate. By opening and closingthe valve during this flow, droplets will be formed according to theopen time and operating frequency of the valve. Thus, in the continuousdispensing mode, the system is not only capable of delivering precisemetered flow rates of reagent, but this can be done with independentcontrol of table speed, reagent concentration per unit length anddroplet size.

If the solenoid valve dispenser is placed very close to the substrate,as shown in FIG. 7 (to the left), then reagent will flow directly ontothe substrate providing a continuous line. This mode of continuousoperation may provide particular advantage where reagent patterns havingvery sharp lines are necessary or desirable. If desired, a continuousdrive reagent pump may also be used to assure a steady flow of reagentto the solenoid valve dispenser. More commonly, however, the solenoidvalve dispenser will be spaced at least slightly away from thesubstrate, as shown in FIG. 7 (to the right). In this mode, discretedroplets will be formed which are ejected onto the substrate to form thedesired pattern. The size of each droplet will determine the effectiveresolution of the resulting pattern formed on the substrate. It isconvenient to express this resolution in terms of dots per inch or“dpi.” The present invention should be capable of achieving dispensingresolutions in the range of 300-600 dpi or higher.

Dot Dispensing

In the dot dispensing mode, individual droplets can be dispensed atpreprogrammed positions. This can be accomplished by synchronizing thesolenoid valve and displacement pump with the X, X-Y or X-Y-Z platform.The metering pump is incremented to create a hydraulic pressure wave.The solenoid valve is coordinated to open and close at predeterminedtimes relative to the pump increment. The valve may be initially openedeither be before or after the pump is incremented. While the valve isopen the pressure wave pushes a volume of fluid down the nozzle forminga droplet at the exit orifice at the time of peak pressure amplitude.The droplet will have a size determined by the incremental volumeprovided by the metering pump. For example, a 50-microliter syringe pumpwith a 12,000 step resolution will provide an incremental displacementvolume of 4.16 nanoliters.

The timing and duration of each valve cycle relative to the hydraulicpressure wave created by the pump can be determined experimentally toachieve stable dispensing operation having the desired droplet size. Ifthe wavelength of the hydraulic pressure wave is too large relative tothe valve open time, the pressure wave may actually force the valveshut. If the wavelength is about equal to or shorter than the valve opentime, then a pulse of fluid will be displaced forming a droplet. Again,the size or volume of the droplet will be determined primarily by theincremental displacement volume of the syringe pump.

If the valve open time is large relative to the pressure wavelength thenseveral pulses or displacements may travel through the valve during thetime in which it is open. This may be acceptable or even desirable forsome applications, such as where bursts of droplets are desired at aprogrammed valve frequency. For example, the dispensing apparatus can beprogrammed to produce 10 drops at 100 Hz to yield a composite drop sizeof about 41.6 nanoliters. This mode of operation can provide the abilityto dispense drop sizes down to less than 1 nanoliter with theappropriate nozzle design. It will depend on the resolution of themetering pump and the minimum valve open/close time and the size of theexit orifice. If the valve is left open too long, however, then thesystem may not maintain enough pressure to eject droplets. To achievethe most stable dispensing operation, the valve open time should beabout consistent with the droplet volume or composite droplet volumedispensed.

The timing, frequency and duty cycle of the solenoid valve relative tothe syringe pump and movable carriage/platform can be coordinated orsynchronized by any one of a number controllers well known in the art.Typical controllers are microprocessor based and provide any one of anumber of output control pulses or electrical signals of predeterminedphase, pulse width and/or frequency. These signals may be used, forexample, to control and coordinate the syringe pump, movablecarriage/platform and solenoid valve dispenser in accordance with thepresent invention.

There may also be some optimum phasing of the pressure pulse relative tothe open/close times of the solenoid valve. Stable operation has beenobserved, for instance, when the valve open time is adjusted to be aneven multiple of the pulse width of the pump increment, with theopen/close time of the valve being synchronized to be in phase with theresulting pressure wave. For example, with a 50-microliter syringe pumpoperating at 12,000-step resolution, the incremental displacement volumewill be about 4.16 nanoliters. Therefore, stable operation should bepossible with droplet sizes of some multiple of 4.16 nanoliters. Theminimum droplet size for stable operation may be increased or decreasedaccordingly by adjusting the resolution of the pump or by increasing thesize of the syringe. For a large droplet, say 9×4.16 nanoliters=33.28nanoliters, it may be preferred to open the valve longer than forsmaller droplets in order to get more uniform lines and stableoperation. Again, the range of stable operation can be readilydetermined experimentally for each desired operating mode.

Aspirating

Another preferred mode of operation is aspirating (“sucking”) precisequantities of reagent or other liquids from a sample or reservoir. Thismode may be used, for example, in a “suck and spit” operation whereby aprecise quantity of fluid is aspirated from one vial containing a samplefluid and then dispensed into another vial or onto a diagnostic teststrip for testing or further processing. The dispenser/aspirator may bea simple nozzle or needle (“asperating tube”) or, more preferably, itmay be a solenoid valve dispenser. The metering pump anddispenser/aspirator are preferably synchronized or coordinated with anX, X-Y or X-Y-Z movable platform.

In operation the metering pump is filled with a wash fluid such asdistilled water. The tip of the dispenser or aspirating tube is placedinto the fluid to be aspirated and the metering pump is decremented todraw a precise quantity of the fluid into the tip of the dispenser oraspirating tube. It is generally desirable to only aspirate a smallvolume of reagent into the tip of the solenoid valve dispenser that doesnot pass into the valve. The metering pump is then incremented todispense a precise portion of the fluid into a receiving receptacle orsubstrate. The remaining fluid is dispensed into a waste/wash receptaclealong with a predetermined quantity of the wash. This ensures that thefluid sample does not get diluted with the wash fluid and the sample isflushed out after each aspirate and dispense cycle.

This mode of operation has particular advantage for dispensing highviscosity reagents. Conventional solenoid valve dispensers typically donot work very well with solutins having a viscosity above about 5centipoise. But there are many applications where it is desirable todispense reagents having high viscosities. Advantageously, the presentinvention, when used in the aspirate/dispense mode, provides a solutionto this problem. Again, in the aspirate/dispense mode the system will befilled with a wash fluid such as water or a water-based solution havinga low viscosity. The reagent is first aspirated then dispensed,followedby washing of the valve by dispensing excess wash fluid.

In the case of a viscous reagent, the present invention can aspirate anddispense such reagents very effectively by decreasing the speed ofaspiration. This allows more time for the more viscous fluid to flowinto the tip of the solenoid valve dispenser or aspirating tube. Becausethe viscous fluid will then be hydraulically coupled to the wash fluid,it can now be dispensed from the nozzle effectively, since the system isdriven by positive displacement and the fluids are incompressible. Usingthis mode, the present invention can dispense reagents of a viscositythat cannot typically be directly dispensed.

Printing

Another possibly desirable mode of operation may be to use the dropdispensing capability of the present invention in conjunction withelectrostatic, dot matrix, or other printing techniques to createprinted patterns, lines and other geometric shapes on a substrate. Inthis case the metering pump may be used as an internal forcing functionto control quantitatively the droplet size of each dot in a matrixpattern. By superimposing programmed dispensing frequency function andselective charging and deflecting of droplets, the present invention canprovide drop-on-demand printing having extended capabilities for finerdot sizes and printing resolution.

For example, a dispensing apparatus 10″ having features of the presentinvention may be in used in conjunction with an electrostatic printinghead 200, such as shown in FIG. 8 to create a dot matrix pattern on asubstrate. The dispensing apparatus can be programmed to dispensedroplets of a predetermined size and frequency pattern. These dropletscan be electrically charged such that they may be deflected by anelectric field generated between a pair of deflector plates 210. Theamount of charge put on a droplet is variable, and thus, the amount ofdeflection is also variable. The electronics may be arranged andadjusted such that droplets can be placed in any number of predeterminedpositions. Selective charging and deflecting of individual droplets maybe used to form a desired dot matrix pattern, as shown. Alternatively,multiple dispensers and pumps may be arranged to form an array ofdrop-on-demand dispensers for simple dot matrix printing operations.

Dispensing Platforms

As noted above, the dispensing apparatus in accordance with the presentinvention may also be mounted on any one of a number of membraneplacement and handling modules. For instance, a single platform 100 maybe used to mount multiple dispensers to handle one or more reagents, asshown in FIG. 9. Such dispensing platforms may be microprocessor-basedand are preferably controlled through an industry standard input/outputI-O controller (not shown), such as an RS232 interface. A remoteprogrammable controller 110 may also be used, as desired, to control thevarious dispensing equipment and platforms or to program a central I/Ocontroller. The invention is also well suited for use with individualmembrane strip handling modules and continuous reel-to-reel handlingmodules. An individual membrane strip module may incorporate an X-Ytable motion for dispensing. The reel-to-reel platform may incorporateconstant-speed membrane transport with mountings attached for motion ofone or more dispensers. A drying oven (not shown) may also be used toincrease production throughput, as desired.

It will be appreciated by those skilled in the art that the methods andapparati disclosed in accordance with the present invention can be usedto dispense a wide variety of liquids, reagents and other substances anda variety of substrates. Although the invention has been disclosed inthe context of certain preferred embodiments, those skilled in the artwill readily appreciate that the present invention extends beyond thespecifically disclosed embodiments to other alternative embodiments ofthe invention. Thus, it is intended that the scope of the inventionshould not be limited by the particular disclosed embodiments describedabove, but should be determined only by a fair reading of the claimsthat follow.

1. A method for dispensing a liquid onto a substrate, comprising:providing first and second positive displacement pumps intermediate aliquid reservoir and a dispenser for substantially continuously meteringof said liquid, said positive displacement pumps being connected inparallel such that each pump is capable of independently drawing saidliquid from said liquid reservoir and independently metering said liquidto said dispenser; metering a predetermined quantity or flow rate ofsaid liquid using one of said first and second positive displacementpumps; supplying said metered quantity or flow rate of said liquid tosaid dispenser to form droplets of a predetermined size and/or qualitywhich are deposited onto said substrate; regulating the metering of saidliquid and operation of said dispenser so that the liquid flow rate,droplet frequency, droplet volume and/or droplet velocity is controlledsubstantially independently of the particular flow characteristics ofsaid liquid; and operating said positive displacement pumps inalternating succession such that while one of said positive displacementpumps meters said liquid to said dispenser the other positivedisplacement pump can draw additional liquid from said liquid reservoirto allow for substantially continuous and uninterrupted dispensing. 2.The method of claim 1, wherein said method further comprises providingrelative motion between said substrate and said dispenser.
 3. The methodof claim 1, wherein said first and second positive displacement pumpsare isolated from one another.
 4. The method of claim 1, wherein saiddispenser comprises a solenoid dispenser.
 5. The method of claim 1,wherein said dispenser comprises a piezoelectric dispenser.
 6. Themethod of claim 1, wherein said droplets are in the range of about 4nanoliters to about 1 microliter.
 7. The method of claim 1, whereinattainable droplet sizes range in size by a factor of greater than about250.
 8. The method of claim 1, wherein at least one of said positivedisplacement pumps comprises a syringe pump.
 9. The method of claim 1,wherein said method further comprises selectively charging anddeflecting said droplets of said liquid to form a pattern on saidsubstrate.
 10. The method of claim 9, wherein said method furthercomprises placing a variable amount of charge on said droplets so thatthe amount of deflection of said droplets is variable.
 11. The method ofclaim 1, wherein said method further comprises electrically chargingsaid droplets such that they are deflected by an electric fieldgenerated between a pair of electrostatic deflector plates.
 12. Themethod of claim 1, wherein said method further comprises using a remoteprogrammable controller to control the dispenser and/or said positivedisplacement pumps.